A Developmental Switch of Gene Expression in the Barley Seed Mediated by HvVP1 (Viviparous-1) and HvGAMYB Interactions

Abstract

The accumulation of storage compounds in the starchy endosperm of developing cereal seeds is highly regulated at the transcriptional level. These compounds, mainly starch and proteins, are hydrolyzed upon germination to allow seedling growth. The transcription factor HvGAMYB is a master activator both in the maturation phase of seed development and upon germination, acting in combination with other transcription factors. However, the precise mechanism controlling the switch from maturation to germination programs remains unclear. We report here the identification and molecular characterization of Hordeum vulgare VIVIPAROUS1 (HvVP1), orthologous to ABA-INSENSITIVE3 from Arabidopsis thaliana. HvVP1 transcripts accumulate in the endosperm and the embryo of developing seeds at early stages and in the embryo and aleurone of germinating seeds up to 24 h of imbibition. In transient expression assays, HvVP1 controls the activation of Hor2 and Amy6.4 promoters exerted by HvGAMYB. HvVP1 interacts with HvGAMYB in Saccharomyces cerevisiae and in the plant nuclei, hindering its interaction with other transcription factors involved in seed gene expression programs, like BPBF. Similarly, this interaction leads to a decrease in the DNA binding of HvGAMYB and the Barley Prolamine-Box binding Factor (BPBF) to their target sequences. Our results indicate that the HvVP1 expression pattern controls the full Hor2 expression activated by GAMYB and BPBF in the developing endosperm and the Amy6.4 activation in postgerminative reserve mobilization mediated by GAMYB. All these data demonstrate the participation of HvVP1 in antagonistic gene expression programs and support its central role as a gene expression switch during seed maturation and germination.

Temporal tissue‐specific regulation of transcriptomes during barley (Hordeum vulgare) seed germination

“…We detected differential expression of several barley transcription factors whose roles in germination have been defined by prior studies using alternative methods, including; HvABI5 (HORVU5Hr1G068230), HvVP1/ABI3 (HORVU3Hr1G092690, GAMYB (HORVU3Hr1G079490), HvSAD/DOF23 (HORVU6Hr1G065250), HvWRKY38 (HORVU6Hr1G028790), HvSLN1 (HORVU4Hr1G006930) and HvFUS3 (HORVU3Hr1G067350) (Gubler et al , ; Casaretto and Ho, ; Isabel‐LaMoneda et al , ; Mare et al , ; Moreno‐Risueno et al , ; Abraham et al , ). The abundance of the HvVP1 transcript, a homolog of the Arabidopsis ABA‐regulatory transcription factor ABI3 , was significantly downregulated at later time points (40 and 48 h) in all three organs, consistent with the localization and abundance of this transcript previously determined by RNA in situ hybridization (Abraham et al , ). Furthermore, two of the MYB transcription factors (HORVU0Hr1G018970, HORVU2Hr1G010450) were homologous to Arabidopsis MYBs that regulate germination and root development (Liberman et al , ; Chen et al , ).…”

“…The abundance of this transcript did not change significantly during germination, indicating that it was present in the scutellum of stored and germinating seeds. Furthermore, ABI3/VP1‐related B3 domain and EIN3‐like transcription factors are known to regulate germination in barley and other plant species (Lopez‐Molina et al , ; Casaretto and Ho, ; Zhang et al , ; Abraham et al , ; Yu et al , ). An AUGMIN subunit‐encoding transcript (HORVU7Hr1G060640) was stored plumule specifically in addition to the transcription factor‐encoding transcripts.…”

“…These included many transcription factors, such as AP2, ERF, homeodomain, TCP, ARF, IAA, bHLH, GATA, zinc finger, MYB, bZIP, and DOF family proteins, as well as more general components such as histone deacetylase, jumonji, cytosine methyltransferase, methyl‐CpG binding, and nuclear transcription factor Y proteins. One of the candidates identified is a well known barley MYB transcription factor, HvGAMYB, expressed in aleurone cells to regulate GA signaling during germination (Gubler et al , ; Abraham et al , ). In addition, barley homologs of Arabidopsis transcription factors involved in germination were also identified as candidate regulators, including HORVU6Hr1G075650 (AT3G47620, AtTCP14), HORVU4Hr1G064120 (AT2G35530, AtbZIP16) and HORVU2Hr1G036710 (AT5G66730, AtIDD1) (Feurtado et al , ; Hsieh et al , ; Rueda‐Romero et al , ; Resentini et al , ).…”

Three Rice NAC Transcription Factors Heteromerize and Are Associated with Seed Size

“…For the promoter of ONAC026 , SN and NB have an additional stretch of 677 bp, holding multiple copies of DOFCOREZM (AAAG) and EBOXBNNAPA (CANNTG, Figure 3B ), a probable explanation for the higher expression levels in these two accessions ( Figures 2A,D ). EBOXBNNAPA, conserved in many seed storage protein promoters is critical for directing seed-specific expression (Ellerstrom et al, 1996; Ravel et al, 2014), while DOFCOREZM is the recognition core of DOF proteins, reported to activate several storage protein genes (Yamamoto et al, 2006; Abraham et al, 2016). For ONAC023 , the promoter sequences for SN and NB are similar, though there are SNPs amongst the two as well, and hence a similar expression pattern though levels are different.…”

“…Regulatory networks are an important aspect of seed development, and often involve formation of homo/heterodimers between TFs. In barley, HvVP1 interacts with HvGAMYB and BPBF and represses their DNA-binding activity (Abraham et al, 2016). Maize OPAQUE2 dimerizes with MADS47 and enhances its activation of zein genes (Qiao et al, 2016).…”